The present invention relates generally to the fabrication of semiconductor devices. In particular, to the dicing of integrated circuit chips with great accuracy, and within close proximity to fabricated elements on the chip. The invention relates, most particularly, to the dicing of silicon sensor chips as employed for digital image sensors.
Image sensor dies for scanning document images, such as Charge Coupled Devices (CCDs), typically have a row or linear array of photo-sites together with suitable supporting circuitry integrated onto silicon. Usually, a die of this type is used to scan line by line across the width of a document with the document being moved or stepped lengthwise in synchronism therewith.
In the above application, the image resolution is proportional to the ratio of the scan width and the number of array photo-sites. Because of the difficulty in economically designing and fabricating long dies, image resolution for the typical die commercially available today is relatively low when the die is used to scan a full line. While resolution may be improved electronically as by interpolating extra image signals, or by interlacing several smaller dies with one another in a non-collinear fashion so as to crossover from one die to the next as scanning along the line progresses, electronic manipulations of this type adds to both the complexity and the cost of the system. Further, single or multiple die combinations such as described above usually require more complex and expensive optical systems.
However, a long or full width array, having a length equal to or larger than the document line and with a large packing of co-linear photo-sites to assure high resolution, has been and remains a very desirable arrangement. In the pursuit of a long or full width array, forming the array by assembling several small dies together end to end has become an exemplary arrangement. However, this necessitates providing dies whose photo-sites extend to the border or edge of the die, so as to assure continuity when the die is assembled end to end with other dies, and at the same time provide edges that are sufficiently smooth and straight to be assembled together without loss of image data.
Although the standard technique of scribing and cleaving silicon wafers used by the semiconductor industry for many years produces dies having reasonably controlled dimensions, the microscopic damage occurring to the die surface during the scribing operation has effectively precluded the disposition of the photo-sites at the die edge. This is because the top surface of silicon wafers is virtually always parallel to the <100> plane of the crystalline lattice so that, when a wafer of this type is cut or diced with a high speed diamond blade, chips and slivers are broken away from the top surface of the wafer in the direct vicinity of the channel created by the blade. This surface chipping typically extends to about 50 microns, thus, rendering it impossible for active elements to be located any closer than about 50 microns from the dicing channel. This as a result, has driven the adoption of V-shaped grooves as a technique for providing much smoother dicing and thereby enabled tighter dicing accuracy and closer proximity of active chip elements to the chip/die edge.
U.S. Pat. No. 4,814,296 discloses a process for forming individual dies having faces that allow the dies to be assembled against other like dies to form one and/or two dimensional scanning arrays wherein the active side of a wafer is etched to form small V-shaped grooves defining the die faces, relatively wide grooves are cut in the inactive side of the wafer opposite each V-shaped groove, and the wafer cut by sawing along the V-shaped grooves, the saw being located so that the side of the saw blade facing the die is aligned with the bottom of the V-shaped groove so that there is retained intact one side of the V-shaped groove to intercept and prevent cracks and chipping caused by sawing from damaging the die active surface and any circuits thereon. U.S. Pat. No. 4,814,296 is hereby incorporated by reference in its entirety for its teaching.
However, utilization of a V-shaped groove technique while effective has proven to be expensive. This expense may be broadly characterized as due primarily to two things. Both of these arise from the requirement for an anisotropic etch so as to maintain a V-groove wall which is parallel to the <111> crystalline plane found in the wafer. First, there are the extra foundry costs. An anisotropic etch is a wet etch and as such is a non-standard process for most silicon foundries. This also means that the wafers must be stripped of their photoresist and require extra handling with placement in an off-line wet etch tool as well. Secondly, there is the cost impact resulting from chip yield effects. Anisotropic etching is by nature an aggressive etch due to the chemicals employed and, thus, often attacks and damages the top layers of passivation oxide and metal on the wafer. This is further exacerbated by the stripping of the photoresist, which would otherwise act as a barrier layer and aid in preventing wafer damage.
Therefore, as discussed above, there exists a need for an arrangement and methodology which will solve the problem of preventing cracks and chipping caused by damage from sawing while minimizing the costs of doing so. Thus, it would be desirable to solve this and other deficiencies and disadvantages as discussed above with an improved semiconductor dicing methodology.
The present invention relates to a method for dicing die from a semiconductor wafer while allowing a very close cut of a die edge relative to active elements on the die without damaging the active elements. The method steps comprise etching a U-groove via a dry etch in the semiconductor wafer and sawing the semiconductor wafer along the U-groove where one edge of the saw is substantially in alignment with the bottom of the U-groove.
In particular, the present invention relates to a method for dicing die from a semiconductor wafer while allowing a very close cut of a die edge relative to active elements on the die without damaging the active elements. The method steps comprising etching by way of a first dry etch an opening down to the surface of the semiconductor wafer, followed by etching by way of a second dry etch a U-groove in the opening down to the surface of the semiconductor wafer created by the first dry etch, and then sawing the semiconductor wafer along the U-groove where one edge of the saw is substantially in alignment with the bottom of the U-groove.
The present invention also relates to a method of fabricating high resolution image sensor dies from a wafer so that the dies have precision faces to enable the dies to be assembled with other like dies to form a larger array without image loss or distortion at the points where the dies are assembled together. The method comprising the steps of etching small U-shaped grooves in one side of a wafer delineating the faces of the dies where the dies are to be separated from the wafer. This is followed by forming grooves in the opposite side of the wafer opposite each of the U-shaped grooves, the axis of the grooves being parallel to the axis of the U-shaped groove opposite thereto. In turn this is followed by, sawing the wafer along the U-shaped grooves with one side of the cut made by sawing being substantially coextensive with the bottom of the U-shaped grooves whereby one side of the U-shaped grooves is at least partially obliterated by the sawing, the sides of the U-shaped grooves that remain serving to prevent development of fractures in the die beyond the remaining side as the wafer is being sawed.